Traditional membranes are used in a wide variety of liquid and gas separation and ion transport technologies. In many circumstances, these membranes lack pore uniformity and robustness under operating conditions that can lead to a decrease in performance. Traditional membranes overcome these limitations by using thick membrane materials that impede transport and/or selective permeation that result in decreased performance and increased operating costs.
In one area of traditional polymeric membrane technology, relevant to separating solvents from dissolved salts, the active-site architecture responsible for ion rejection consists of a thick, greater than 100 nm, and compressible polymer coating having non-robust and non-uniform pore architectures. The non-uniformity means that many pores are non-functional. The thickness of the membranes causes resistance to transport. Due to this resistance, the membrane requires high applied pressure to concentrate salty solution on one side of the membrane and force purified solvent to the other side. This thin-film composite membrane design for liquid deionization has remained unchanged for over 30 years.
Considerable research has been undertaken to improve the polymer-based thin-film composites. This research has resulted in incremental progress only, motivating a search for alternative materials demonstrating both high flux and selectivity. Zeolite-based membranes have been considered, but these show high resistance to flow. Membranes composed of stacked, oriented-carbon nanotubes demonstrate high permeabilities around 5.5 cm/hr per bar of applied pressure, more than an order-of-magnitude improvement over thin-film composites, but fail in terms of salt rejection in solutions of relevant ionic strength and are complicated to fabricate.
Several problems existing in the prior art membranes have been solved in cellular membranes. Combined high flux and selectivity of cellular membranes is achieved by membrane-bound ion and molecular channels, whose pore size and chemistry is defined with sub-nanometer precision through protein folding. The thickness of these cellular membranes is limited to that of the cellular membrane bilayer, which is only about 4 nm thick. Functional biological channels are uniform and robust under physiologically relevant operating conditions. However, cellular membranes cannot withstand large pressure differences across the membrane, which are common in the operating conditions of desalination membranes.
FIG. 1 shows an aquaporin channel 100, which is a cellular membrane found in kidney and other cells. Aquaporins allow water transport at fast rates on the order of 109 molecules per pore per second and complete rejection of ions. The active portion of the channel is a thin 2 nm region composed primarily of hydrophobic, aromatic residues (one phenylalanine, and several tryptophan side-chains) and polar groups from the protein backbone that line the open pore permeation pathway. The narrowest segment of the channel has a constriction between 0.3 nm and 0.5 nm in diameter. The open pore architecture is supported and stabilized by the surrounding protein and liquid environment.
What is needed to achieve low resistance, highly selective permeation is a selectively permeable membrane that mimics a cellular membrane, which is porous, reliable, robust, and that can be made thin with uniform small-diameter pores having a desired chemistry. A porous membrane demonstrating these qualities would have numerous applications that would benefit from reduced operation costs and improved performance.